Seismicity and earthquake hazard in the UK

The UK is not a country generally associated in the
public mind with earthquakes. However, while the UK is nowhere near
in the same league as high seismicity areas such as California and
Japan, it nevertheless has a moderate rate of seismicity, sufficiently
high to pose a potential hazard to sensitive installations such as
dams and chemical plants. As a result, the last twenty years have
seen a large effort by BGS and others to improve instrumental monitoring
of earthquakes and to research and revise the historical seismicity.
This research has made it possible to calculate the level of hazard
in the UK rather more accurately than hitherto. Here I give a synopsis
of present knowledge regarding UK seismicity and present an illustrative
seismic hazard map. (Note: in this paper terms describing earthquake
size should be read as relative to UK conditions; ie a "large" earthquake
for the UK is around magnitude 5 or more.)

In considering the pattern of British seismicity,
we can first look briefly at the history of the investigations of
British earthquakes that have produced the present state of knowledge.

The study of British earthquakes has in the past
been somewhat neglected compared to some other countries, not necessarily
those with more active seismicity. For example, in Norway, a complete
archive of contemporary macroseismic investigations ( = studies of
felt effects) of Norwegian earthquakes exists, that goes back to the
1870s. In the UK, historically, investigation of earthquakes has generally
been in the hands of self-appointed investigators of semi-amateur
status. Prior to 1889, such investigations were one-off affairs. Thus
the 1863 Hereford earthquake was surveyed by EJ Lowe, while the 1871
Appleby earthquake did not attract scientific attention.

EJ Lowe, an early Victorian investigator of earthquakes

Between 1889 and 1926, systematic macroseismic investigations
were conducted by Charles Davison, although with an increasing proportion
of earthquakes passed over by him towards the end of this period.
Davison's publications are generally lacking in terms of presenting
his original data, and the tens of thousands of questionnaires he
gathered during his career seem to have been destroyed after his death.

Charles Davison, amateur seismologist

After Davison, the principal macroseismic investigator
was ATJ Dollar, whose attention to the subject was somewhat erratic;
for example, he made a study of the 1946 Lochaber earthquake but not
the larger 1944 Skipton earthquake.

Up to the 1970s, the most recent publication attempting
to survey the whole history of British earthquakes was still Davison's
catalogue of 1924 even though this was now 50 years out of date.

Modern instrumental monitoring of British earthquakes
began around 1970 with the establishment of LOWNET by the Global Seismology
Group of BGS (then IGS) which has subsequently expanded to the present
country-wide monitoring network, supported by a customer group led
by the Department of the Environment. This led to routine macroseismic
surveying of British earthquakes from 1974 onwards, and the beginnings
of reinvestigation of historical seismicity at about the same time
by Roy Lilwall.

In the early 1980s, the expansion of the nuclear
power programme in the UK led to increased activity in revaluating
historical seismicity both from macroseismic and instrumental records,
and major studies were made by several investigators independently,
including BGS, Imperial College London, and private consultancies.
All this work was combined and synthesised in the early 1990s to make
a consistent, numerate earthquake catalogue for the UK, which was
published by BGS in 1994.

A map of earthquakes in the UK

Above we see a map of earthquakes in the UK, taken
from the BGS catalogue. It is clear from this map that the spatial
distribution of earthquakes is neither uniform nor random. For example,
in Scotland most earthquakes are concentrated on the west coast, between
Ullapool and Dunoon, with the addition of centres of activity near
the Great Glen at Inverness and Glen Spean, and a small area around
Comrie, Perthshire, and extending south to Stirling and Glasgow. The
Outer Hebrides, the extreme north and most of the east of Scotland
are virtually devoid of earthquakes. For the north-west of Scotland
the absence of early written records, the small population, and the
recent lack of recording instruments means that there may be a data
gap; for instance, there are indications that an earthquake occurred
in 1925, possibly near Ullapool, with magnitude probably about 3½
ML, for which there are no first-hand reports. However, many other
parts of Scotland, especially south of the Highland line, are quite
well-documented, at least since 1600, and therefore the lack of earthquakes
is genuine.

Further south a similar irregularity is seen. If
one draws a quadrilateral from Penzance to Holyhead to Carlisle to
Doncaster, most English and Welsh earthquakes will be included within
it. The northeast of England seems to be very quiet; almost aseismic.
The southeast has a higher rate of activity, with a number of earthquakes
which seem to be "one-off" occurrences. The most notable example of
these is the 1884 Colchester earthquake, a magnitude 4.6 ML event
which was the most damaging British earthquake in at least the last
400 years, and yet which occurred in an area (Essex) otherwise more
or less devoid of earthquakes from the earliest historical period
up to the present day.

Earthquake damage at Langenhoe, Essex, in 1884

There are also important centres of activity near
Chichester and Dover. The former produced a swarm-like series of small,
high-intensity earthquakes in the 1830s and was active again in 1963
and 1970.

Offshore, there is significant activity in the English
Channel and off the coast of Humberside. Because only the larger events
in these places are likely to be felt onshore, the catalogue in the
pre-instrumental period is probably under-representative of the true
rate of earthquake activity in these zones. Even after the introduction
of seismometers, offshore earthquakes may still have gone unnoticed
on account of the distance to the nearest instruments. The Central
Grabens of the North Sea are now known to be active features, only
because of the improvements in instrumental monitoring over the last
fifteen years.

Newspaper report of the 1931 Dogger Bank earthquake

The whole of Ireland is practically free of earthquakes.
This is clearly a real phenomenon and not a product of reporting -
one writer, as early as the 17th century, remarks in describing an
earthquake (probably Welsh) felt in Dublin in 1534, that an earthquake
is such a rare thing in Ireland that when it happens it is considered
a wonder.

Certain centres can be identified as showing typical
patterns of activity. For example, the Caernarvon area of north-west
Wales is one of the most seismically active places in the whole UK.
Both large and small earthquakes, usually accompanied by many aftershocks,
occur at regular intervals. The most recent of these larger events
was the earthquake of 17 July 1984 (5.4 ML), which was one of the
largest ever UK earthquakes to have an epicentre on land and had a
very protracted aftershock sequence. Two further felt earthquakes
have occurred there since, on 29 July 1992 (3.5 ML) and 10 February
1994 (2.9 ML). It is tempting to ascribe several early earthquakes
of unknown epicentre (eg that of 20 February 1247) to this area just
because it seems to be such a favoured site for large earthquakes.

In South Wales, on the other hand, although a line
of epicentres of significant events can be traced from Pembroke (an
earthquake in 1892) to Newport (active in 1974), only the Swansea
area shows consistent recurrence, with significant earthquakes occurring
in 1727, 1775, 1832, 1868 and 1906. (Given this periodicity it may
be that a further earthquake in this area is due in the near future.)
The Hereford-Shropshire area has also produced large earthquakes in
1863, 1896, 1926 and 1990, but none of these share a common epicentre.

The area of the Dover Straits is particularly significant
because of the occurrence there of two of the largest British earthquakes
in 1382 and 1580 (both of magnitude about 5¾ ML). Since 1580 the only
earthquakes there have been much smaller, raising the question of
whether there is a danger of another 1580-style earthquake in the
near future. The area may be structurally continuous with a zone of
activity running east through Belgium, in which case it could be argued
that stress in this area since 1580 has been released further east.
This does not rule out another 1580-type earthquake in the future,
but it is impossible to estimate how soon it might occur.

In the north of England seismic activity occurs over
a more or less continuous area from Leicester to Carlisle. The most
prominent centres of repeating activity here are the upper end of
Wensleydale (with significant earthquakes in 1768, 1780, 1871, 1933
and 1970) and to a lesser extent the Skipton area.

What is remarkable is the lack of correlation between
this pattern and the structural geology of the UK.

Tectonic sketch map of the UK

This map shows the major crustal subdivisions in
the UK. The boundaries between areas of moderate or high seismicity
and areas of very low seismicity do not correspond to any major structural
feature; for instance the sharp dividing line running SE from Inverness.
And the major boundaries shown above are not clearly reflected in
the pattern of seismicity either as dividing lines between zones of
differing rates of seismicity nor as lineations marked by earthquakes.
It seems likely that the pattern of seismicity may be influenced by
the distribution of ice during the last glaciation - certainly for
Scotland this appears to be the case.

It has long been realised that larger earthquakes
occur less frequently than smaller earthquakes, the relationship being
exponential, ie roughly ten times as many earthquakes larger than
4 ML occur in a particular time period than earthquakes larger than
magnitude 5 ML. This can be expressed by the Gutenberg-Richter formula

log N = a - b M

where N is the number of earthquakes per year exceeding
a given magnitude M. The constant a reflects the absolute level of
seismicity in an area, and the value of b has generally been found
to be consistently close to 1.0.

Magnitude frequency plot for the UK

This holds true for the UK. The graph above shows
an analysis for the area 10o W to 2o E and 49o N to 59o N. This deliberately
excludes the northern North Sea area which is of high seismicity and
completely under-represented in the catalogue before 1970 because
of the impossibility of detecting smaller events in this area before
that date. A least-squares regression to this data gives the relationship

log N = 3.82 - 1.03 M

Also shown is an alternative doubly-truncated exponential
model which gives a curved fit ot the data at the higher magnitude
end.

One can therefore draw the following conclusions
about average recurrence - the UK may expect:

Seismic hazard calculations in regions of low seismicity,
such as the UK, are generally based on probabilistic methodology.
Probabilistic seismic hazard assessment (PSHA) uses a combination
of interpreted geological and seismological data to calculate the
probability that a certain level of ground motion will be exceeded,
or not exceeded, in a given period of time.

This methodology can be divided into three principal
components as follows:

(i) Definition of a set of seismic source zones which
define the geographical variation of earthquake activity. These source
zones are based on the distribution of observed seismic activity together
with geological and tectonic factors and represent areas where the
seismicity is assumed to be homogenous; ie there is an equal chance
that a given earthquake will occur at any point in the zone.

(ii) An understanding of earthquake recurrence with
respect to earthquake magnitude, as described in the previous section.

(iii) An attenuation relationship is required which
defines what ground motion should be expected at Location A due to
an earthquake of known magnitude at Location B. The rate at which
the strength of shaking decreases with distance from an earthquake's
epicentre varies regionally and has to be calculated or estimated.
Peak ground acceleration (pga) is the measure of earthquake shaking
most used by engineers in this country. However, it has two disadvantages
- firstly, the attenuation of pga in the UK is very poorly known,
and secondly, pga is actually not a particularly good measure of the
actual expectation of damage. A useful alternative is intensity, which
is an expression of ground shaking in terms of its effects. The attenuation
of intensity in the UK is very well documented, and intensity is directly
proportional to damage, making it a very meaningful parameter.

The intensity attenuation model used here is expressed
by the formula

I = 3.32 + 1.44 ML - 3.34 log R

where ML is local magnitude and R is hypocentral
distance in kilometres.

Seismic hazard studies in the UK in the past have
been mostly single-site studies for particular installations. The
first attempt to look at hazard for the UK as a whole using the PSHA
methodology was conducted by Ove Arup around 1991. This study calculated
hazard at eleven representative sites in the UK. Following this, a
study to produce contour maps of UK seismic hazard was commissioned
by the Department of Trade and Industry, and was carried out by BGS
and AEA Technology. In this study the computer code SUNMIC was used,
which allows a "logic tree" model to be applied to the hazard, by
which uncertainty in input parameters can be modelled by the inclusion
of multiple choices each with a weighting value).

Seismic hazard results

Here is a sample hazard map of the UK, based on the
study made for the DTI.

Intensity hazard map for the UK

The map shows intensities that are 90% likely not
to be exceeded in 50 years - equivalent to a return period of 475
years. For guidance, a simplified equivalence of the intensity values
(on the European Macroseismic Scale) is as follows:

3 - Felt by few

4 - Felt by many indoors, windows and doors rattle

5 - Felt by most indoors, small objects fall over

6 - People run out in alarm, slight damage to buildings (plaster
cracks)

7 - Moderate damage to buildings (chimneys fall, cracks in walls)

As might be expected, the areas of highest hazard
parallel the areas where earthquakes have been most common in the
past, but particularly those places where repeated earthquake activity
has been highly localised - this localisation has a pronounced effect
on the hazard calculations compared to areas where the seismicity,
while high, is more diffuse and less repetitive. The zones where hazard
is higher than average encompass the W Highlands of Scotland, an arcuate
zone running from Carlisle to Pembroke, NW Wales and W Cornwall. The
places in the UK with lowest seismic hazard are Northern Ireland (especially
the western counties) and outlying parts of Scotland, including the
Orkneys and Outer Hebrides.

The actual values of hazard are not particularly
high, since the predicted intensity for the higher zones is only 6
EMS. In other words, even in areas of relatively high exposure to
earthquakes in the UK, if a facility has a life of 50 years there
is only a 10% chance that it will experience shaking equivalent to
intensity 6. Moving briefly from hazard to risk, if we take as a guideline
that probably less than 5% of buildings of normal construction (eg
conventional brick houses) will be damaged in a place when the intensity
there is 6, the probability of damage for a single house in 50 years
is therefore less than 0.5%.